Position detection technique applied to proximity exposure

ABSTRACT

The exposure surface of a wafer to be exposed is formed with a wafer mark having edges for scattering incident light. The edge of the wafer mark has a curved portion whose image vertically projected upon a plane in parallel to the exposure surface has a curved shape. The surface of an exposure mask is formed with a mask mark having edges for scattering incident light. The edge of the exposure mask has a curved portion whose image vertically projected upon a plane in parallel to the surface of the exposure mask has a curved shape. The wafer and exposure mask are juxtaposed with and spaced apart by a gap from the exposure surface of the wafer. Illumination light is applied to the curved portions of the edges of the wafer and mask marks. Light scattered from the wafer and mask marks is observed along a direction oblique to the exposure surface to thereby detect a relative position of the wafer and exposure mask.

This application is a Division of U.S. patent application Ser. No.09/031,184, filed Feb. 26, 1998, now U.S. Pat. No. 6,049,373 issued onApr. 11, 2000.

This application is based on Japanese Patent Applications No. 9-46525,9-46526, and 9-46527 all filed on Feb. 28, 1997 and No. 9-253786 filedon Sep. 18, 1997, the entire contents of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to position detection techniques usingscattered light from edges or apexes, and more particularly to aposition detection method, a position detection apparatus and analignment mark, suitable for improving throughput of proximity exposure.

b) Description of the Related Art

As the position alignment of a wafer and a mask by using an alignmentsystem composed of a lens system and an image processing system, avertical detection method and an oblique detection method are known. Thevertical detection method observes alignment marks along a directionvertical to the mask surface, whereas the oblique detection methodobserves alignment marks along a direction oblique to the mask surface.

A chromatic bifocal method is known which is used as a focussing methodfor the vertical detection method. With this chromatic bifocal method, amask mark formed on a mask and a wafer mark formed on a wafer areobserved with light having different wavelengths, and focussed on thesame flat plane by utilizing chromatic aberration of the lens system.This method can principally set an optical resolution of a lens high, sothat an absolute position detection precision can be made high.

However, since alignment marks (the mask marks and the wafer marks) areobserved along the vertical direction, the observing optical systementers an exposure region. If exposure is performed in this state, theoptical system intercepts exposure light. It is therefore required toretract the optical system from the exposure region when exposure isperformed. The time required for the optical system to retract from theexposure region lowers throughput. Further, during the exposure, thealignment marks cannot be observed and their positions cannot bedetected. This may cause a low alignment precision during the exposure.

With the oblique detection method, the optical system is disposed withits optical axis being set oblique to the mask surface so that it can belocated at the position not intercepting exposure light. It is thereforeunnecessary to retract the optical system during exposure, and alignmentmarks can be observed even during exposure. It is possible to preventposition misalignment during exposure, without lowering throughput.

With this oblique detection method, however, wafer and mask marksobserved obliquely are focussed so that an absolute precision ofposition detection is lowered by image distortions. Furthermore, sincethe optical axis of illumination light is not coincident with theoptical axis of observation light, it is not possible to coaxiallydispose both axes. Therefore, the illumination light axis becomes easyto shift from an ideal optical axis. As the illumination light axisshifts from the ideal optical axis, images are distorted and correctposition detection becomes difficult.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide position detectiontechniques capable of providing high precision alignment withoutlowering throughput, while allowing position detection during exposure.

According to one aspect of the present invention, there is provided aposition detection method comprising the steps of: disposing a waferhaving an exposure surface and an exposure mask, with a gap being setbetween the exposure surface and the exposure mask, the wafer having awafer mark formed on the exposure surface, the wafer mark having edgesfor scattering incident light, each edge having a curved portion whoseimage vertically projected upon a plane in parallel to the exposuresurface has a curved shape, the exposure mask having a mask mark formedon the surface thereof, the mask mark having edges for scatteringincident light, each edge having a curved portion whose image verticallyprojected upon a plane in parallel to the surface of the exposure maskhas a curved shape; and detecting a relative position of the wafer andthe exposure mask by applying illumination light to the curved portionsof the edges of the wafer and mask marks and by observing, along adirection oblique to the exposure surface, light scattered from thecurved portions.

Since light scattered from edges is observed obliquely, the observingoptical system can be configured not to enter the exposure area. It isnot necessary to retract the optical system during the exposure and itis possible to observe light scattered from edges even during theexposure. Since the edge has a curved portion, a variation in the shapesand positions of edges to be caused by the influences of manufactureprocess variation can be reduced.

According to another aspect of the present invention, there is provideda semiconductor substrate having an exposure surface formed with aplurality of aligning wafer marks along a direction perpendicular to aplane of incident of incident light, each wafer mark having an edge forscattering the incident light, and an image of the edge verticallyprojected upon the exposure surface having at least a curved portion.

According to another aspect of the present invention, there is providedan exposure mask having a plurality of aligning mask marks disposedalong a direction perpendicular to a plane of incident of incidentlight, each mask mark having an edge for scattering the incident light,and-an image of the edge vertically projected upon the surface of theexposure mask having at least a curved portion.

Since an edge has a curved portion, a variation in the shapes andpositions of edges to be caused by the influences of manufacture processvariation can be reduced. Since a plurality of edges are disposed alonga direction perpendicular to an incident surface, a plurality of imagescan be observed at the same time. By moving these images in parallel andsuperposing the images one upon another, a relative position can beeasily detected.

According to another aspect of the present invention, there is provideda position detection method comprising the steps of: disposing a memberwith an exposure surface to be exposed and an exposure mask, with a gapbeing set between the exposure surface and the exposure mask, the memberhaving an alignment mark formed on the exposure surface, the alignmentmark having edges or apexes for scattering incident light, the exposuremask having a mask mark formed on the surface thereof, mask mark havingedges or apexes for scattering incident light; and detecting a relativeposition of the member and the exposure mask by applying illuminationlight to the edges or apexes of the alignment and mask marks, byfocussing light scattered from the alignment and mask marks on a lightreception plane, and by observing images on the light reception plane,whereby a light flux scattered from one of the alignment and mask marksis attenuated or light fluxes scattered from both of the alignment andmask marks are attenuated differently so that a light intensity of animage formed by the light flux scattered from the alignment mark andfocussed on the light reception plane becomes near to a light intensityof an image formed by the light flux scattered from the mask mark andfocussed on the light reception plane.

Edges or apexes of the alignment and mask marks allow the scatteredlight to be observed, because an image is formed by a flux of scatteredlight in the aperture of an objective lens of the observing opticalsystem. Further, even if illumination light is applied along a directionallowing only scattered light from edges to be incident, edge scatteredlight can be observed because normal reflection light from the alignmentand mask marks are not incident upon the observing optical system.

Since the intensities of light scattered from the alignment and maskmarks are made near to each other, image signals corresponding to imageson a focal plane can be obtained at a high S/N ratio.

According to another aspect of the present invention, there is provideda position detection apparatus comprising: an illumination opticalsystem for applying illumination light to a member having an exposuresurface to be exposed and to a mask disposed in parallel to the exposuresurface of the member and spaced by a gap from the exposure surface; andan observation optical system having an optical axis oblique to theexposure surface of the member, for focussing light scattered from themember and the mask onto a light reception plane, the observationoptical system including an optical filter disposed just in front of thelight reception plane, and a transmission factor of the optical filteris different between an area corresponding to an area where lightscattered from the member is focussed and an area corresponding to anarea where light scattered from the mask is focussed.

By properly selecting transmission factors of two areas of an opticalfilter, the intensities of images on the focal plane formed by lightscattered from the alignment and mask marks are made near to each other.Accordingly, image signals corresponding to images on a focal plane canbe obtained at a high S/N ratio.

According to another aspect of the present invention, there is provideda position detection apparatus comprising: an illumination opticalsystem for applying illumination light to a wafer and a mask juxtaposedwith and spaced by a gap from the wafer; a first observation opticalsystem having an observation optical axis oblique to an exposure surfaceof the wafer, for focussing light reflected or scattered from the waferand the mask upon a first light reception plane; and a secondobservation optical system for focussing light reflected from a partialreflection mirror disposed along the optical axis of the firstobservation optical system, upon a second light reception plane, thesecond observation optical system having a magnification factordifferent from a magnification factor of the first observation opticalsystem.

Since reflected or scattered light is observed obliquely, the opticalsystem is not necessary to be disposed in the exposure area.Accordingly, it is not necessary to retract the observing optical systemduring the exposure and the position detection is always possible evenduring the exposure. It is possible to perform coarse position alignmentby using one of the first and second observing optical systems having alower magnification factor, and to perform fine position alignment byusing the other optical system having a higher magnification factor.

According to another aspect of the present invention, there is provideda position detection method comprising the steps of: disposing a waferwith an exposure surface and a mask, with a gap being set between theexposure surface and the exposure mask, the wafer having a wafer markformed on the exposure surface, the wafer mark having edges or apexesfor scattering incident light, the mask having a mask mark having edgesor apexes for scattering incident light; coarsely detecting a relativeposition of the wafer and the mask by applying illumination light to theedges or apexes of the wafer and mask marks and by observing lightscattered from the wafer and mask marks with a first observing opticalsystem having an observation optical axis oblique to the exposuresurface; moving at least one of the wafer and the mask in accordancewith the detection results obtained at the coarse relative positiondetecting step to coarsely align the wafer and the mask; finelydetecting the relative position of the wafer and the mask by observinglight scattered from the wafer and mask marks with a second observingoptical system having the same observation optical axis as the firstobserving optical system and having a higher magnification factor thanthe first observing optical system; and moving at least one of the waferand the mask to finely align the wafer and the mask, in accordance withthe detection results obtained at the fine relative position detectingstep.

Prior to the fine position alignment, coarse position alignment isperformed by using a lower magnification factor optical system.Accordingly, a high position alignment precision is not required whenthe wafer and mask are first held.

According to another aspect of the present invention, there is provideda position detection method comprising the steps of: disposing a firstmember with a main surface and a second member juxtapose with and spacedby a gap from the main surface, the first member having a firstalignment mark formed on the main surface for scattering incident light,and the second member having a second alignment mark for scatteringincident light; focussing illumination light applied to the first andsecond marks and scattered therefrom onto a light reception plane via aconverging optical system having an optical axis oblique to the mainsurface, whereby light receiving elements are disposed on the lightreception plane in a matrix shape and the row direction of the lightreception plane corresponds to a direction of a cross line between themain surface and a virtual plane perpendicular to the optical axis ofthe converging optical system; deriving a first partial two-dimensionalimage and a second partial two-dimensional image respectively similar toa reference pattern from an image formed by light scattered from thefirst mark and focussed on the light reception plane and from an imageformed by light scattered from the second mark and focussed on the lightreception plane; forming a synthesized one-dimensional image signal byaccumulating in the column direction image signals of pixels in a firstarea including the first partial two-dimensional image and in a secondarea including the second partial two-dimensional image; and obtaining arelative position of the first and second marks in accordance with thesynthesized one-dimensional image signal.

According to another aspect of the present invention, there is provideda position detection apparatus comprising: illumination means forapplying illumination light to a first member having a main surfaceformed with a first alignment mark for scattering incident light and toa second member having a second alignment mark for scattering incidentlight, the second member being held facing the surface formed with thesecond mark toward the main surface; image detecting means having alight reception plane, on which pixels are disposed in a matrix shape,said image detecting means generating image signals corresponding to thepixels in accordance with intensity of light the pixels receive;converging optical system having an optical axis oblique to the mainsurface for focussing light scattered from the first and second marksonto the light reception plane; reference pattern storage means forstoring a reference pattern; and control means for deriving a firstpartial two-dimensional image and a second partial two-dimensional imageeach similar to a reference pattern stored in the reference patternstorage means, from an image formed by light scattered from the firstmark and focussed on the light reception plane and from an image formedby light scattered from the second mark and focussed on the lightreception plane, forming a synthesized one-dimensional image signal byaccumulating in the column direction the image signals of the pixels ina first partial area including the first partial two-dimensional imageand in a second partial area including the second partialtwo-dimensional image, and obtaining a relative position of the firstand second marks in accordance with the synthesized one-dimensionalimage signal.

Since scattered light is observed obliquely, it is not necessary todispose a converging optical system in the areas where the first andsecond marks are formed. Accordingly, the areas of the first and secondmarks can be exposed without retracting the converging optical system.By accumulating image signals in the column direction, an S/N ratio canbe improved. Since accumulation is performed for only partial areas, theaccumulation calculation time can be shortened.

According to another aspect of the present invention, there is provideda position detection method comprising the steps of: focussing twoimages on a light reception plane having light receiving elementsdisposed in a matrix shape for generating an image signal correspondingto incident light, the two images being formed at different positions inthe column direction of the light reception plane; accumulating, in thecolumn direction, image signals of pixels in two areas each of whichpartially overlaps at least a portion of each of the two images; andobtaining a relative position of the two images in accordance with animage signal accumulated at the accumulating step.

According to another aspect of the present invention, there is provideda position detection apparatus comprising: a light reception planehaving light receiving elements disposed in a matrix shape forgenerating an image signal corresponding to incident light; and controlmeans for determining two areas each of which partially overlaps atleast a portion of each of two images formed on the light receptionplane, accumulating the image signals of pixels in the two areas in thecolumn direction, and obtaining a relative position of the two images inaccordance with an image signal obtained through the accumulation.

Accumulation in the column direction improves the S/N ratio. Sinceaccumulation is performed for only partial areas, the accumulationcalculation time can be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of a position detectionapparatus used by first and second embodiments of the invention and alsoused by proposals made by the present inventor in the past, FIG. 1B is aplan view of wafer and mask marks, FIG. 1C is a diagram showing imagesof edges of the wafer and mask marks shown in FIG. 1B formed by lightscattered from the edges, and a light intensity distribution on theimage plane, and FIG. 1D is a schematic cross sectional view of thewafer and mask surfaces near at an object plane.

FIGS. 2A, 2C and 2E are perspective views of wafer marks, and FIGS. 2Band 2D show images formed on a focal plane.

FIG. 3 is a plan view showing wafer and mask marks having apexes fromwhich illumination light is scattered.

FIGS. 4A and 4B are respectively plan views and perspective viewsshowing edge patterns constituting a wafer mark according to the firstembodiment of the invention.

FIG. 5 is a diagram showing a light intensity distribution of an imageof one edge of the wafer mark shown in FIG. 4A formed by light scatteredtherefrom.

FIGS. 6A and 6B are plan views showing examples of the layouts of waferand mask marks according to the first embodiment of the invention.

FIG. 7A is a plan view of wafer and mask marks, FIG. 7B is a crosssectional view taken along one-dot chain line B2—B2 of FIG. 7A, and FIG.7C is a cross sectional view taken along one-dot chain line C2—C2.

FIG. 8 is a sketch of images of wafer and mask marks shown in FIG. 7Aand formed by light scattered from the marks.

FIGS. 9A and 9B are graphs showing image signals illustrating the secondembodiment in which wafer marks are made of different materials.

FIG. 10A is a cross sectional view showing a region near a focal planeof the position detection apparatus according to the second embodimentof the invention, and FIG. 10B is a front view of an optical filtershown in FIG. 10A.

FIG. 11 is a diagram showing the fundamental structure of a positiondetection apparatus used by third and fourth embodiments of theinvention.

FIG. 12A is a schematic plan view showing a light reception plane withimages of wafer and mask marks formed by light scattered from the marks,and FIG. 12B is a diagram showing a reference pattern used by the fourthembodiment.

FIG. 13 is a graph showing one-dimensional synthesized image signalsobtained by the fourth embodiment.

FIG. 14 is a cross sectional view taken along one-dot chain line C2—C2shown in FIG. 7A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to describing the embodiments of this invention, proposals made bythe present inventor in the past (Japanese Patent Applications No.7-294485 (JP-A 139333), and U.S. patent application Ser. No. 08/640,170,the entire contents of which are incorporated herein by reference) willbe described.

FIG. 1A is a schematic cross sectional view of a position detectionapparatus used by the embodiments of the invention and also used whenthe above proposals were made by the present inventor. The positiondetection apparatus is constituted of a wafer/mask holder unit 10, anoptical system 20 and a controller 30.

The wafer/mask holder unit 10 includes a wafer holder 15, a mask holder16, and a drive mechanism 17. For the position alignment, a wafer 11 isheld on the upper surface of the wafer holder 15, and a mask 12 is heldon the lower surface of the mask holder 16. The wafer 11 and mask 12 aredisposed in parallel, with a preset gap being set between the exposuresurface of the wafer 11 and the surface (mask surface) of the mask 12 onthe wafer side. On the exposure surface of the wafer 11, positionaligning wafer marks 13 are formed, and on the mask surface of the mask12, a position aligning mask mark 14 is formed.

The wafer and mask marks 13 and 14 have edges or apexes for scatteringincident light. Light incident upon these marks are scattered by theedges or apexes and regularly reflected by other areas. Regularreflection means the reflection that most components of incident lightare reflected in the same direction.

The drive mechanism 17 can produce a relative motion of the wafer holder15 and mask holder 16. By defining an X-axis as directing in thedirection from the left to right as viewed in FIG. 1A, a Y-axis asdirecting in the direction vertical to the drawing sheet from the frontto back thereof, and a Z-axis as directing in the direction normal tothe exposure surface, the relative motion of the wafer 11 and mask 12 ispossible in the X-axis, Y-axis and Z-axis directions, in a rotationdirection (θ_(z) direction) about the Z-axis, and in rotation (flap)directions (θ_(x) and θ_(y) directions) about the X-axis and Y-axisdirections.

The optical system 20 includes an image detector 21, a lens 22, a halfmirror 23 and a light source 24. The optical axis 25 of the opticalsystem 20 is set in parallel to the X-Z plane and oblique to theexposure surface. Although a single lens is shown in FIG. 1A, aplurality of lenses may be used or a relay lens may be used whennecessary.

Illumination light radiated from the light source 24 is reflected by thehalf mirror 23, forming a light flux along the optical axis 25 which ismade obliquely incident upon the exposure surface via the lens 22. Thelight source 24 is positioned at the focal point of the lens 22 on theimage side so that the illumination light radiated from the light source24 is collimated by the lens 22 into a parallel light flux (parallelpencil of rays). The intensity of illumination light can be adjusted atthe light source.

Of the light scattered by the edges or apexes of the wafer and maskmarks 13 and 14, light incident upon the lens 22 is converged by thelens 22 and focussed on a light reception plane of the image detector21. Illumination by the optical system 20 is therefore telecentricillumination which provides the same optical axis for both illuminationand observation.

The image detector 21 photoelectrically converts images of the wafer andmask marks 13 and 14 formed by light scattered from the edges or apexesand focussed on the light reception plane, into image signals which areinput to the controller 30.

The controller 30 processes the image signals input from the imagedetector 21 to detect a relative position of the wafer and mask marks 13and 14. The controller 30 sends a control signal to the drive mechanism17 to make the wafer and mask marks 13 and 14 have a predeterminedrelative position. In accordance with this control signal, the waferholder 15 or mask holder 16 is moved.

FIG. 1B is a plan view showing a relative position of the wafer and maskmarks 13 and 14. One mark is constituted of three rectangular patternsdisposed in the X-axis direction, each side of the rectangular patternbeing in parallel to the X- or Y-axis. As will be later described, threeor more rectangular patterns may be disposed. The mask mark 14 isdisposed between a pair of wafer marks 13.

The wafer and mask marks 13 and 14 are shown in FIG. 1A as crosssectional views taken along one-dot chain line A1—A1 shown in FIG. 1B.Illumination light incident upon the wafer and mask marks 13 and 14 isscattered by the edge of each rectangular pattern which is shown in FIG.1B and projects along the optical axis. Light incident upon the areaother than the edges is regularly reflected and is not incident upon thelens 22. The image detector 21 can therefore detect only the lightscattered from the edges.

Next, the characteristics of an image formed by edge scattered lightwill be described.

A light intensity distribution I(x, y) of an image formed by incoherentmonochromatic light is given by the following equation (1):

I(x, y)=∫∫O(x−x′, y−y″)·PSF(x′, y′) dx′dy′  (1)

where the coordinates (x, y) represent a position on the surface of anobservation object, O(x, y) represents an intensity distribution oflight reflected from the observation object, PSF(x, y) represents a(point spread function) of a lens, and integral is performed over thewhole surface of the observation object.

If attention is drawn to one edge of each rectangular pattern shown inFIG. 1B, this edge may be considered as light reflecting fine pointsdisposed in the Y-axis. The intensity distribution of light reflectedfrom such a fine point is assumed as having a Dirac delta function δ. Itis practical to approximate the intensity distribution of lightscattered from a fine point, to a delta function. Assuming that the edgeextends in the Y-axis within the range where the isoplanatism of thelens is satisfied, O(x, y) can be replaced by δ(x). Therefore, theequation (1) can be represented by the following equation (2):$\begin{matrix}\begin{matrix}{{I(x)} = \quad {\int{\int{{{\delta \left( {x - x^{\prime}} \right)} \cdot {{PSF}\left( {x^{\prime},y^{\prime}} \right)}}{x^{\prime}}{y^{\prime}}}}}} \\{= \quad {\int{{{PSF}\left( {x,y^{\prime}} \right)}{y^{\prime}}}}}\end{matrix} & (2)\end{matrix}$

where I(x) represents a line spread function of a lens, which is givenby:

I(x)=LSF(x)  (3)

where LSF(x) represents the line spread function of the lens.

If illumination light has continuous spectra, I(x) can be given by:

I(x)=∫LSFλ(x−Δxλ)dλ  (4)

where λ is a wavelength of light, LSFλ represents the line spreadfunction at the wavelength λ, Δxλ represents a lateral shift amount of aline image caused by lens chromatic aberration at the wavelength λ, andintegral is performed over the whole wavelength range.

It can be understood from the equation (4) that observation of lightscattered from an edge is equivalent to observation of the line spreadfunction of the lens. A stable image can therefore be obtained alwaysthrough the observation of light scattered from the edge, without beinginfluenced by the in-plane intensity distribution of light reflectedfrom the observation object.

The left area of FIG. 1C shows the shape of an image formed by scatteredlight which was on the light reception plane of the image detector 21shown in FIG. 1A. By defining the x-axis as directing in the direction(corresponding to the X-axis direction of the exposure surface) of across line between the plane of incident including the observationoptical axis and the light reception plane, and the y-axis as directingin the direction (corresponding to the Y-axis of the exposure surface)perpendicular to the x-axis of the light reception plane, an imageformed by one edge has a straight line shape parallel to the y-axis. Theimage of each mark therefore has a shape configured by three straightlines which are parallel to the y-axis and juxtaposed in the x-axisdirection.

Between a pair of images 13A of the wafer marks 13 formed by edgescattered light, an image 14A of the mask mark 14 formed by edgescattered light appears. Since the observation optical axis is obliqueto the exposure surface, the image 14A of the mask mark and the images13A of the wafer marks are detected at different positions along thex-axis direction.

The right area of FIG. 1C shows a light intensity distribution, alongthe y-axis direction, of the wafer mark images 13A and the mask markimage 14A. A distance in the y-axis direction between one wafer markimage 13A and the mask mark image 14A is represented by y1, and adistance in the y-axis direction between the other wafer mark image 13Aand the mask mark image 14A is represented by y2. By measuring y1 andy2, it is possible to detect a relative position of the wafer and maskmarks 13 and 14 shown in FIG. 1B in the Y-axis direction, i.e., in thedirection perpendicular to the plane of incident of illumination light.

For example, if the mask mark is to be positioned at the center betweenthe pair of wafer marks in the Y-axis direction, one of the wafer andmask is moved relative to the other to make y1 and y2 have the samevalue. Position alignment in the Y-axis direction shown in FIG. 1B istherefore possible in this manner. Position alignment in the X-axis,Y-axis and θ_(z) direction can be performed by providing three sets ofthe alignment marks and optical systems shown in FIGS. 1A and 1B. InFIG. 1A, although the illumination and observation axes are setcoaxially, it is not necessarily required to be coaxial. It issufficient if scattered light only is incident upon an object lens ofthe observation system and normal reflection light is not incident.

Next, a method of measuring a gap between the exposure surface and themask surface will be described. In an object space of the opticalsystem, light scattered from a plurality of points on one flat planeperpendicular to the optical axis 25 is focussed at the same time on thelight reception plane of the image detector 21. A plane, defined by aset of points in the object space focussed on the light reception plane,is called an “object plane”.

Light scattered from each edge or apex of the wafer and mask marks onthe object plane is focussed on the light reception plane, and lightscattered from each edge or apex not on the object plane is not madein-focus and is defocussed more as the edge or apex becomes more apartfrom the object plane. Therefore, the scattered light image of an edgeor apex positioned nearest to the object plane becomes clearest, and theimage of another edge or apex at the position more remote from the edgeor apex nearest to the object plane becomes more vague.

A distance x, shown in FIG. 1C is a distance in the x-axis directionbetween points in the highest in-focus state of the wafer mark image 13Aand mask mark image 14A. Namely, the distance x₁ is generally equal tothe distance between points in the highest in-focus state of the waferand mask marks vertically projected upon the plane of incident.

FIG. 1D is a schematic cross sectional view of the wafer and masksurfaces 11 and 12 on the plane of incident near at the object plane. Apoint Q₂ is a point on a cross line between the wafer surface 11 andobject plane, and a point Q₁ is a point on a cross line between the masksurface 12 and object plane. The length of a line segment Q₁-Q₂corresponds to the distance x₁ shown in FIG. 1C.

If the length L of the line segment Q₁-Q₂ is represented by L(Q₁, Q₂),the gap δ between the exposure surface 11 and mask surface 12 is givenby:

δ=L(Q₁, Q₂)×sin(α)  (5)

where α is an angle of the optical axis 25 relative to the directionnormal to the exposure surface 11. The gap δ can be known by calculatingthe length of the line segment Q₁-Q₂ from the measured distance x₁. Inorder to obtain a precise gap δ, it is preferable to measure thedistance x₁ precisely. To this end, it is preferable that the depth offocus is shallow. It is also preferable to dispose a number ofrectangular patterns shown in FIG. 1B in the X-axis direction.

A target value of the distance x₁ is stored in advance in the controller30, and the drive mechanism 17 is controlled in such a manner that themeasured distance x₁ takes the target value. In this manner, a desiredgap can be set between the exposure and mask surfaces 11 and 12.

FIG. 2A is a perspective view showing an example of one rectangularpattern of the wafer mark. Illumination light is made obliquely incidentalong the oblique optical axis in the X-Z plane shown in FIG. 2A, andlight scattered from the edge extending in the Y-axis direction isobserved. In this case, since the image formed by scattered light hasthe intensity distribution given by the equation (4), an image long inone direction extending along the y-axis direction of the lightreception plane can be obtained as shown in FIG. 2B. This image has theline spread function of a lens.

FIG. 2C shows another pattern having a shorter length in the Y-axisdirection than the rectangular pattern shown in FIG. 2A. If the lengthof an edge is shorter than the lens resolution, the intensitydistribution O(x, y) of reflected light in the equation (1) may possiblybe replaced by δ(x, y). Therefore, the equation (1) is changed to:$\begin{matrix}\begin{matrix}{{I\left( {x,y} \right)} = \quad {\int{\int{{{\delta \left( {{x - x^{\prime}},{y - y^{\prime}}} \right)} \cdot {{PSF}\left( {x^{\prime},y^{\prime}} \right)}}{x^{\prime}}{y^{\prime}}}}}} \\{= \quad {{PSF}\left( {x,y} \right)}}\end{matrix} & (6)\end{matrix}$

where PSF(x, y) is a point spread function of a lens.

If illumination light has continuous spectra, I(x, y) is given by:

I(x, y)=∫PSFλ(x−Δxλ, y−Δyλ)dλ  (7)

where λ is a wavelength of light, PSFλ represents the point spreadfunction at the wavelength λ, Δxλ represents a lateral shift amount inthe x-axis direction of a point image caused by lens chromaticaberration at the wavelength λ, Δyλ represents a lateral shift amount inthe y-axis direction of a point image caused by lens chromaticaberration at the wavelength λ, and integral is performed over the wholewavelength range.

By setting the length of an edge equal to or shorter than the resolutionof a lens, it is possible to obtain a point image approximated to thepoint spread function of a lens shown in FIG. 2D.

FIG. 2E is a perspective view of a rectangular pattern wherein light isscattered from a position near at the apex on which three planes areintersected. An image formed by light scattered from a position near atthe apex shown in FIG. 2E can presumably be approximated to the pointspread function given by the equations (6) and (7). In thisspecification, a unit of a pattern having an edge or apex from whichlight is scattered is called an edge pattern.

FIG. 3 shows an example of the layout of mask and wafer marks havingapexes from which illumination light is scattered. A mask mark 62 isdisposed between wafer marks 52A and 52B. Each of the alignment marks52A, 52B and 62 is formed by disposing an edge pattern of a square planeshape on three rows in the X-axis direction at a pitch P and on twocolumns in the Y-axis direction. One apex of each edge pattern of asquare plane shape is directed to the positive X-axis direction, i.e.,toward the observation optical system.

By using the layout of edge patterns shown in FIG. 3 having apexes fromwhich illumination light is scattered and by observing light scatteredfrom the apexes, position alignment of a wafer and a mask can beachieved also by the method described with FIGS. 1A to 1C.

Next, the position detection method according to the first embodiment ofthe invention will be described. The oblique detection optical systemused by the first embodiment is the same as that shown in FIG. 1A.Position alignment through observation of light scattered from a linearedge has been described with reference to FIGS. 1B and 2A, and positionalignment through observation of light scattered from an apex has beendescribed with reference to FIGS. 2E and 3. In this embodiment, light isobserved which is scattered from an edge whose image verticallyprojected upon the exposure surface or mask surface is a curved line.

FIG. 4A is a plan view of a wafer mark having curved edges, and FIG. 4Bis a perspective view of the wafer mark. This wafer mark is formed bypatterning a Ta₄B film deposited on an SiC layer formed on a siliconsubstrate surface, and is constituted of three edge patterns having amesa structure of about 3 μm length and about 1 μm width. Opposite endsof each edge pattern have a semicircular shape with a radius ofcurvature of about 0.5 μm. An X-ray mask can be formed by removing thesilicon substrate at a window region through etch-back and by leavingthe SiC layer.

In observing edge scattered light from the wafer mark shown in FIG. 4A,the silicon substrate is held by the wafer holder 15 shown in FIG. 1A insuch a manner that the longitudinal direction of the edge pattern, thein-plane direction perpendicular to the longitudinal direction, and thesubstrate normal direction become coincident with the X-, Y- and Z-axesshown in FIG. 1A. Light scattered from the curved edge having a radiusof curvature of about 0.5 μm is then observed.

FIG. 5 shows the measurement results of a light intensity distributionof scattered light from one curved edge of the wafer mark shown in FIGS.4A and 4B, as observed with the oblique detection optical system shownin FIG. 1A. The ordinate represents a light intensity, and the abscissarepresents the Y-axis on the silicon substrate surface. Namely, theabscissa direction corresponds to the y-axis shown in FIG. 1C, the widthdirection corresponds to the X-axis. The magnification factor of thelens of the observation optical system was 100, the numerical aperture(NA) of the object lens was 0.35, and the illumination light was while.

As shown in FIG. 5, the image of scattered light from the curved edgehas generally a point-like shape and takes a maximum light intensitygenerally at one point. Namely, an image similar to that of scatteredlight from an apex of a wafer mark can be obtained.

The position of the apex such as shown in FIG. 2E is susceptible to theinfluences of manufacture process variation when the wafer mark isformed. In contrast, the position of the curved edge is not susceptibleto the influences of manufacture process variation, so that highprecision position alignment can be performed stably. In the exampleshown in FIGS. 4A and 4B, each edge of the wafer mark has a semicircularshape. This shape may be any other gentle curve so long as an image ofgenerally a point-like shape can be obtained. In forming a wafer markhaving such a gentle curve, the outer periphery of an edge pattern of awafer mark is not necessarily required to be gentle. For example, aphotomask having a rectangular window may be used and the outerperiphery of the resist pattern is made smooth by utilizing diffractionof exposure light.

The radius of curvature of each edge of the wafer mark shown in FIGS. 4Aand 4B was about 0.5 μm which was generally the same as the lensresolution. By setting the radius of curvature of each edge generallyequal to or smaller than the resolution, an image of edge scatteredlight becomes almost a point-like shape. It is therefore preferable toset the radius of curvature of each edge generally equal to or smallerthan the lens resolution, in order to perform high precision positionalignment. This does not imply that an edge having a radius of curvaturelarger than the lens resolution cannot be used. It is preferable to beempirically determined which radius of curvature is suitable forobtaining a desired position alignment precision, by using edges ofvarious shaped.

FIGS. 6A and 6B show examples of the layouts of mask and wafer marks. Inthese two examples, two wafer marks 40A and 40B and two wafer marks 42Aand 42B are disposed in the Y-axis direction, and mask marks 41 and 43are disposed between the wafer marks 40A and 40B and between the wafermarks 42A and 42B, respectively.

Each of the alignment marks 40A, 40B and 41 shown in FIG. 6A isconstituted of edge patterns having a circular plane shape long in theX-axis direction disposed on two rows in the X-axis direction and on twocolumns in the Y-axis direction.

Each of the alignment marks 42A, 42B and 43 shown in FIG. 6B isconstituted of edge patterns having a circular plane shape disposed onthree rows in the X-axis direction and on two columns in the Y-axisdirection.

By using the layout of edge patterns with gentle curve shown in FIGS. 6Aand 6B and observing light scattered from such edges, position alignmentof a wafer and a mask can be achieved also by the method described withFIGS. 1A to 1C. The alignment marks shown in FIGS. 6A and 6B areconfigured to allow one alignment mark to be superposed on anotheralignment mark, by moving the marks in parallel to each other afterposition alignment. It is therefore easy to measure the distance betweenalignment marks in the Y-axis direction. By disposing a plurality ofedge patterns in the X-axis direction at a predetermined pitch, itbecomes easy to focus some edge among a plurality of edge patterns andstably detect the relative position. For example, the relative positionof the wafer and the mask can be detected using the same alignment marksand the same position detection apparatus even if the gap between thewafer and the mask changes. Further, the relative position can bedetected while the gap is changing. The gap between a wafer and a maskcan be detected by the method described with FIG. 2D.

FIG. 7A is a plan view showing another example of a relative position ofwafer marks 13A and 13B and a mask mark 14. Each of the wafer marks 13Aand 13B is constituted of rectangular patterns disposed in a matrixform, three patterns in the Y-axis direction and fourteen patterns inthe X-axis direction. The mask mark 14 is constituted of similarrectangular patterns disposed in a matrix form, three patterns in theY-axis direction and five patterns in the X-axis direction. In the stateafter the position alignment is completed, the relative position isestablished in which the mask mark 14 is positioned at generally thecenter in the Y-axis direction between the wafer marks 13A and 13B.

The longer side of each rectangular pattern of the wafer marks 13A and13B and the mask mark 14 is parallel to the X-axis, and the shorter sidethereof is parallel to the Y-axis. The longer side of each rectangularpattern is 2 μm long and the shorter side is 1 μm long. The pitchbetween rectangular patterns of each mark is 4 μm both in the X- andY-axis directions. The distance between centers of the wafer marks 13Aand 13B is 56 μm.

FIG. 7B is a cross sectional view taken along one-dot chain line B2—B2shown in FIG. 7A. For example, the wafer marks 13A and 13B are formed bypatterning an SiN film, a polysilicon film or the like formed on theexposure surface, and the mask mark 14 is formed by patterning a Ta₄Bfilm formed on the mask surface of a membrane 12 made of SiC or thelike.

FIG. 7C is a cross sectional view taken along one-dot chain line C2—C2shown in FIG. 7A. Illumination light incident upon the wafer and maskmarks 13A, 13B and 14 along the optical axis 25 is scattered by theshorter side edge of each rectangular pattern shown in FIG. 7C. Lightincident upon the area other than the edges is regularly reflected andis not incident upon the lens 22 shown in FIG. 1A. The image detector 21can therefore detect only the light scattered from the edges.

Light scattered from points on the object plane 27 of the optical system20 shown in FIG. 1A is focussed at the same time on the light receptionplane of the image detector 21.

In FIG. 7C, light scattered from each edge of the wafer and mask marks13A, 13B and 14 on the object plane 27 is focussed on the lightreception plane, and light scattered from each edge not on the objectplane is not made in-focus and is defocussed more as the edge becomesmore apart from the object plane. Therefore, the scattered light imageof an edge positioned nearest to the object plane becomes clearest, andthe image of another edge at the position more remote from the edgenearest to the object plane becomes more vague.

FIG. 8 is a sketch of images on the light reception plane formed bylight scattered from edges. A u-axis shown in FIG. 8 corresponds to thedirection of a cross line between the object plane 27 shown in FIG. 7Cand the X-Z plane, and a v-axis corresponds to the Y-axis shown in FIG.7C. Images 40A and 40B formed by light scattered from the wafer marks13A and 13B appear separately along the v-axis direction, and an image41 formed by light scattered from the mask mark 14 appears between theimages 40A and 40B.

Since light scattered from the front and back edges of each rectangularpattern is observed, two point-like images appear for each rectangularpattern. Images formed by light scattered from the edges near at theobject plane 27 shown in FIG. 7C are clear, and images formed by lightscattered from edges more apart from the edges near at the object plane28 becomes more vague. Furthermore, since the observation axis 25 isoblique to the exposure surface, the position of the images 40A and 40Bin the highest in-focus state formed by light scattered from the wafermarks is not coincident in the u-axis direction with the position of theimage 41 in the highest in-focus state formed by light scattered fromthe mask mark.

By moving the wafer holder 15 and mask holder 16 shown in FIG. 1A toposition the image 41 formed by light scattered from the mask mark atthe center between the images 40A and 40B formed by light scattered fromthe wafer mark, along the Y-axis direction, position alignment of thewafer 11 and mask 12 can be performed.

Since the position detection apparatus shown in FIG. 1A obliquelyobserves the wafer and mask marks, it is not necessary to set theoptical system 20 in the exposure range so that the optical system 20 isnot required to be retracted during exposure. It is also possible toalways detect the position while the wafer is exposed after the positionalignment. Still further, since the illumination and observation axesare coaxial, there is no axial shift and an image can be obtained alwaysstably.

The second embodiment of the invention will be described next.

FIGS. 9A and 9B show examples of image signals obtained by the imagedetection apparatus 21 shown in FIG. 1A. The abscissa corresponds to thev-axis shown in FIG. 8, and the ordinate represents to a lightintensity. Image signals were obtained by scanning in the v-axisdirection and after each v-axis scan by moving the scan position by apredetermined distance in the u-axis direction. Of these image signals,the image signals shown in FIG. 8 were obtained by scanning the images40A and 40B at the position in the highest in-focus state and byscanning the image 41 at the position in the highest in-focus state.

The image signals shown in FIG. 9A were obtained with the wafer marksmade of polysilicon, and the image signals shown in FIG. 9B wereobtained with the wafer marks made of SiN. The mask marks were both madeof Ta₄B.

As shown in FIGS. 9A and 9B, three peaks corresponding to the mask markappear at generally the central area, and three peaks corresponding tothe wafer mark appear on both sides of the mask mark peaks.

An example of a method of detecting a relative position of a mask markand a wafer mark by using the waveforms shown in FIGS. 9A and 9B will bebriefly described. While the peak waveforms of the mask mark are shiftedin the v-axis direction, correlation factors between the peak waveformsof the mask mark and the peak waveforms of the two wafer marks arecalculated. The distance between the centers of the wafer and mask markscorresponds to the shift amount at which the largest correlation factorwas calculated.

Position alignment of the wafer and mask in the Y-axis direction shownin FIG. 1A can be performed by moving the wafer and mask holders to makeequal the distances from the peak waveforms of the mask mark to the peakwaveforms of the wafer marks on both sides of the mask mark.

In order to improve the calculation precision of correlation factor, itis preferable to use photoelectric conversion elements disposed on thelight reception plane, in the approximately linear area of theinput/output characteristics of the elements, and to obtain imagesignals of high S/N ratio. From these viewpoints, it is preferable tomake the height of waveform peaks of both the mask and wafer marksgenerally equal. However, light scattered from a wafer mark and lightscattered from a mask mark have generally different intensities becauseof difference of materials of the membrane 12 and difference betweenmaterials of wafer and mask marks shown in FIG. 7C, or other reasons.Therefore, as shown in FIGS. 9A and 9B, the heights of peaks of the maskmark are different from the heights of peaks of the wafer mark.

By utilizing the method of the second embodiment of the invention, aheight difference of peaks between the mask and wafer marks can bereduced. With reference to FIGS. 10A and 10B, the position detectionmethod and apparatus according to the second embodiment will bedescribed.

FIG. 10A is a cross sectional view showing the region near a lightreception plane 21 a of the position detection apparatus shown in FIG.1A. An optical filter 26 is disposed just in front of the lightreception plane 21 a perpendicular to the optical axis 25. An imageformed by light scattered from the wafer mark 13 is focussed in an area21 b of the light reception plane 21 a, and an image formed by lightscattered from the mask mark 14 is focussed in an area 21 c. Thefocussing areas may be reversed depending upon the optical system.

A transmission factor of the optical filter 26 is different at the areas26 b and 26 c corresponding to the areas 21 b and 21 c respectively.FIG. 10B is a front view of the optical filter 26. Areas 26 b and 26 chaving different transmission factors are defined in the upper and lowerhalves of a circular glass plate. Even if the intensities of lightscattered from the wafer and mask marks and transmitted through the lens22 are different, the intensities of two scattered light fluxes focussedupon the light reception plane 21 a can be made near to each other byproperly setting the transmission factors of the two areas of theoptical filter 26.

As the intensities of light scattered from the wafer and mask marks andfocussed upon the light reception plane 21 a are made near to eachother, the heights of peaks of the image signals shown in FIGS. 9A and9B corresponding to the mask and wafer marks can be made near to eachother. It is therefore possible to detect the position more precisely.The optical filter 26 is preferably a filter having the characteristicsthat optical paths of light scattered from the wafer and mask marks donot become different.

The oblique detection optical system does not use light diffraction. Itis therefore preferable to use white light in order to avoid theinfluence of light interference. Therefore, the optical filter 26 ispreferably a neutral density filter having a small wavelength dependencyupon transmission factor. This neutral density filter is also convenientin that a transmission factor of each of a plurality of divided areas ofone filter can be easily changed.

It is preferable that the optical filter 26 is set along the opticalaxis to the position where the light flux scattered from the wafer markis perfectly separated from the light flux scattered from the mask mark.Therefore, the optical filter may be disposed not just in front of thelight reception plane 21 a, but at any position so long as two scattedlight fluxes are perfectly separated.

In FIGS. 10A and 10B, light scattered from the mask mark and lightscattered from the wafer mark are focussed via the areas of the opticalfilter having different transmission factors. One scattered light fluxmay be attenuated by the filter and the other scattered light flux maybe focussed without passing through the filter. In this case, a filteris disposed only in one of the areas 21 b and 21 c shown in FIG. 10A.

In the second embodiment, position detection is performed by calculatingcorrelation factors of one-dimensional image signals.

The relative position of a mask and a wafer may be calculated by usingtwo-dimensional image signals shown in FIG. 8. In this case, patternmatching of similarity between the mask and wafer mark images isperformed by moving the image signals in parallel to the u-axis andv-axis directions. With this pattern matching of the two-dimensionalimage signals, a distances between images in the u- and v-axisdirections can be calculated.

Next, a method of measuring a distance between a wafer and a mask willbe described. In FIG. 8, a position u₀, where the images 40A and 40Bformed by light scattered from the wafer marks provide the highestin-focus state in the u-axis direction, corresponds to a cross line P₀between the object plane 27 and the exposure surface shown in FIG. 7C.Also in FIG. 8, a position u₁, where the image 41 formed by lightscattered from the mask mark provides the highest in-focus state in theu-axis direction, corresponds to a cross line P₁ between the objectplane 27 and the mask surface shown in FIG. 7C. For example, a distancebetween the positions u₀ and u₁ can be obtained through pattern matchingof the two-dimensional images shown In FIG. 8.

By representing the length of the line segmenBt P₀-P₁ by L(P₀, P₁), thegap δ between the wafer 11 and mask 12 is given by:

δ=L(P₀, P₁)×sin(α)  (8)

where α is an angle of the optical axis 25 relative to the directionnormal to the exposure surface. The gap δ can therefore be calculatedfrom the length of the line segment P₀-P₁ by measuring the distanceL(u₀, u₁) between the positions u₀ and u₁ in the u-axis direction shownin FIG. 8. In order to calculate the gap δ more precisely, it ispreferable to correctly measure the distance between the positions u₀and u₁ in the U-axis direction. To this end, the depth of focus of thelens is preferably shallow.

Without performing pattern matching between two observed images, eachobserved image may be pattern matched with a reference image. In thiscase, reference image signals are formed on the assumption that a waferand a mask are disposed to satisfy a desired relative position, andstored in a memory in advance. By performing pattern matching ofsimilarity between an observed wafer mark and a pre-stored referencewafer mark, a shift amount of the observed wafer mark from a referenceposition is obtained. Similar to the wafer mark, a shift amount of amask pattern from a reference position is obtained. From these shiftamounts, a relative position between the wafer and mask can be known.

Next, the third embodiment of the invention will be described.

FIG. 11 is a schematic cross sectional view of a position detectionapparatus according to the third embodiment of the invention. Theposition detection apparatus of this embodiment is constituted of awafer/mask holder unit 110, an optical system 120 and a controller 130.

The wafer/mask holder unit 110 includes a wafer holder 115, a maskholder 116, and drive mechanisms 117 and 118. For the positionalignment, a wafer 11 is held on the upper surface of the wafer holder115, and a mask 12 is held on the lower surface of the mask holder 116.The wafer 11 and mask 12 are disposed in parallel, with a preset gapbeing set between the exposure surface of the wafer 11 and the surface(mask surface) of the mask 12 on the wafer side. On the exposure surfaceof the wafer 11, position aligning wafer marks are formed, and on themask surface of the mask 12, a position aligning mask mark is formed.

The drive mechanism 117 can move the wafer holder 115 or mask holder 116to change a relative position of the wafer 11 and mask 12 on theexposure surface. The drive mechanism 118 can move the wafer holder 115to change the gap between the exposure surface of the wafer 11 and themask surface of the mask 12. By defining an X-axis as directing in thedirection from the left to right as viewed in FIG. 11, a Y-axis asdirecting in the direction vertical to the drawing sheet from the frontto back thereof, and a Z-axis as directing in the direction normal tothe exposure surface, the drive mechanism 117 adjusts the relativeposition of the wafer 11 and mask 12 in the X-axis and Y-axis directionsand in a rotation direction (θ_(z) direction) about the Z-axis, whereasthe drive mechanism 118 adjusts the relative position of the wafer 11and mask 12 in the Z-axis direction and in rotation (flap) directions(θ_(x) and θ_(y) directions) about the X-axis and Y-axis directions.

The optical system 120 includes image detectors 121A and 121B, lenses122 and 128, half mirrors 123 and 126A, an optical fiber 124, and amirror 126B. The optical axis 125 of the optical system 120 is set inparallel to the X-Z plane and oblique to the exposure surface.

Illumination light radiated from the optical fiber 124 is reflected bythe half mirror 123, forming a light flux along the optical axis 125which is made obliquely incident upon the exposure surface via the lens122. The illumination light transmitted through the lens is changed to aparallel light flux (parallel pencil of rays).

The illumination light is scattered from a scattering area such as anedge and apex of the wafer and mask marks formed on the wafer 11 andmask 12. Of the scattered light, light incident upon the lens 122 isconverged by the lens 122 and a fraction thereof transmits through thehalf mirrors 123 and 126A and is focussed on a light reception plane129A of the image detector 121A. A magnification factor of an imagefocussed upon the light reception plane 129A is, for example, 20. Of thescattered light, light reflected by the half mirror 126A is reflected bythe mirror 126B, converged by a relay lens 128, and focussed upon alight reception plane 129B of the image detector 121B. A magnificationfactor of an image focussed upon the light reception plane 129B is, forexample, 80 to 100. The two observation systems having differentmagnification factors are disposed in the above manner.

The image detectors 121A and 121B photoelectrically convert imagesformed by light scattered from the wafer 11 and mask 12 and focussed onthe light reception planes 129A and 129B, into image signals which areinput to the controller 130.

The controller 130 processes the image signals supplied from the imagedetectors 121A and 121B by referring to reference patterns stored in areference pattern memory 131 to detect a relative position of the wafer11 and mask 12 in the Y-axis direction.

The relative positions of the wafer 11 and mask 12 in the X- and Y-axisdirections and in the θ_(z) direction can be detected by disposing thetwo optical systems having an oblique optical axes parallel to the X-Zplane and one optical system having an oblique optical axis parallel tothe Y-Z plane.

Control signals are supplied to the drive mechanisms 117 and 118 to makethe wafer 11 and mask 12 have a predetermined relative position. Inaccordance with the control signal, the drive mechanism 117 moves thewafer holder 115 in parallel to the X-Y plane to rotate it about theZ-axis. In accordance with the control signal, the drive mechanism 118moves the wafer holder 115 in parallel to the Z-axis direction toslightly rotate it about the X- and Y-axes.

Position alignment of the wafer 11 and mask 12 shown in FIGS. 7A to 7Cwill be described by using the position detection apparatus shown inFIG. 11.

Light scattered from points on an object plane of the optical system 120shown in FIG. 11 is focussed at the same time upon the light receptionplanes 129A and 129B of the image detectors 121A and 121B.

The depth d of focus of the converging optical system 120 is given by:

d=λ/NA²  (9)

where NA is a numerical aperture of the object lens of the convergingoptical system 120, and λ is a wavelength of illumination light. Lightscattered from edges positioned in the depth d of focus centered to theobject plane 27 is focussed upon the light reception planes 129A and129B of the image detectors 121A and 121B. As shown in FIG. 7A, aplurality of edges of the wafer marks 13A and 13B and the mask mark 14are disposed along the X-axis direction, so that light scattered fromsome of these scattering areas is focussed upon the light receptionplanes. In FIG. 7C, light scattered from an edge outside of the depth dof focus is not made in-focus on the light reception plane, and theimages formed by light scattered from the areas more spaced from theobject plane 27 become more out-of-focus.

The relative position of the wafer and mask can be known similarly tothe second embodiment described with reference to FIGS. 7A to 9B.

A precision of position alignment in the Y-axis direction of theposition detection apparatus shown in FIG. 11 becomes more severe as theintegration degree of semiconductor integrated circuit devices becomeshigher. For example, in the case of a dynamic RAM having a storagecapacity of 16 G bits, a position alignment precision of about 12.5 nmis required.

In order to perform position alignment by using the image signals shownin FIG. 9A or 9B, an error of the relative position of the wafer andmask when the image signals are detected is preferably set within somerange. However, it is difficult to hold the mask 12 by the mask holder116 shown in FIG. 11 at a precision within this error range, while thewafer 11 is held by the wafer holder 115. It is therefore preferable tofirst perform coarse alignment to realize a precision within this errorrange, after the wafer 11 and mask 12 are held.

Such coarse alignment can be performed easily based upon the imagesignals of images of a low magnification factor focussed upon the lightreception plane 129A. After the completion of this coarse alignment,fine alignment at a higher precision is performed based upon the imagesignals of images of a high magnification factor focussed upon the lightreception plane 129B. Since the coarse alignment is performed prior tothe fine alignment, the position alignment precision required when thewafer and mask are held, can be lowered.

As the integration degree of semiconductor integrated devices becomeshigh, the gap between the wafer 11 and mask 12 is required to have acertain precision. For example, in the case of X-ray exposure of 0.1 μmline width, this gap is about 10 to 20 μm and a precision of +/−1 μm isrequired. The gap between the wafer and mask is detected based on theimage signals of images of a low magnification factor focussed on thelight reception plane 129A.

The controller 130 is provided with two image signal processing units,one for processing image signals detected with the optical system forfine alignment and the other for processing image signals detected withthe optical system for coarse alignment. If the gap between the waferand mask is adjusted by using image signals detected with the finealignment optical system, a high speed performance of fine alignment issacrificed because of the processing capacity of the controller 130.This high speed performance of fine alignment can be prevented frombeing lowered, by adjusting the gap between the wafer and mask inaccordance with image signals detected with the coarse alignment opticalsystem.

Next, the fourth embodiment of the invention will be described.

FIG. 12A is a sketch of images on the light reception plane 129A or 129Bformed by light scattered from the edges of the wafer and mask marksshown in FIG. 7A and observed with the position detection apparatusshown in FIG. 11. This sketch is similar to that shown in FIG. 8. Theelevational direction (column direction) in FIG. 12A corresponds to thedirection of a cross line between the object plane 27 shown in FIG. 7Cand the X-Z plane, and the lateral direction (row direction) correspondsto the Y-axis direction shown in FIG. 7C, i.e., the direction of a crossline between a virtual plane perpendicular to the optical axis 125 andthe exposure surface. Photoelectric conversion elements are disposed ina matrix form on the light reception planes 129A and 129B, and eachphotoelectric element at each pixel generates an image signalcorresponding to a received light amount.

The uppermost pixel row in FIG. 12A is called a first row, and the n-thupper pixel row is called an n-th row. The leftmost pixel column in FIG.12A is called a first column and the n-th left pixel row is called ann-th column. The image detectors 129A and 129B of the embodiment eachhave pixels of 498 rows and 768 columns.

A method of detecting a relative position of the wafer marks 13A and 13Band mask mark 14 in accordance with the images 40A, 40B and 41 formed onthe light reception planes, will be described.

The mask and wafer are held by the mask holder 116 and wafer holder 115shown in FIG. 11, and the height of the wafer is adjusted to have apredetermined gap to the mask. In this case, the relative position ofthe mask and wafer can be adjusted coarsely, and the relative positionin the rotation direction about the Z-axis can be adjusted coarsely.

In the following description, it is assumed that FIG. 12A shows theimages of a low magnification factor focussed on the light receptionplane 129A of the image detector 121A. The controller 130 reads areference pattern from the reference pattern memory 131.

An example of the reference pattern is shown in FIG. 12B. The referencepattern is formed based upon the images formed by light scattered fromedges in the depth d of focus centered to the object plane 27 shown inFIG. 7C.

An image similar to the reference pattern is derived from the images onthe light reception plate 129A. The derived images include: partialimages 42A and 42B containing some portions of the images 40A and 40B ofthe wafer marks 13A and 13B; and a partial image 43 containing someportion of the image 41 of the mask mark 14. Next, of the partialtwo-dimensional images 42A and 42B of the wafer mark and the partialtwo-dimensional image 43 of the mask mark respectively focussed on thelight reception plane, the coordinates values are calculated. Inaccordance with the coordinate values, the relative position of thewafer marks 13A and 13B and mask mark 14 can be detected.

In accordance with this detection results, the drive mechanism 117 shownin FIG. 11 is driven to move the wafer so that the mask mark 14 ispositioned at the center between the wafer marks 13A and 13B in theY-axis direction. Coarse alignment in the Y-axis direction at aprecision of about +/−0.5 μm can be performed in the above manner.

Next, fine alignment at a higher precision is performed by using theimages of a high magnification factor detected with the image detector121B.

In the following description, it is assumed that FIG. 12A shows theimages of a high magnification factor focussed on the light receptionplane 129B of the image detector 121B. The controller 130 reads areference pattern for the high magnification factor, from the referencepattern memory 131.

An example of the reference pattern for the high magnification factor isshown in FIG. 12B. Similar to the coarse alignment, this referencepattern is formed based upon the images formed by light scattered fromedges present in the depth d of focus of the high magnification factorimage detector 121B as centered to the object plane 27 shown in FIG. 7C.

An image similar to the reference pattern is derived from the images onthe light reception plate 129B. Partial two-dimensional images 42A and42B of the wafer mark similar to the reference pattern are derived fromthe images 40A and 40B of the wafer marks 13A and 13B, whereas a partialtwo-dimensional image 43 of the mask mark similar to the referencepattern is derived from the image 41 of the mask mark 14. If images havesmall out-of-focus degrees and images similar to the reference image canbe derived, such images formed by light scattered from edges outside ofthe depth d of focus may be used to form the reference pattern, inaddition to the images formed by light scattered from edges in the depthd of focus.

Consider now the case wherein the partial two-dimensional images 42A and42B of the wafer mark are positioned in the range from the a-th row tob-th row and the partial two-dimensional image 43 of the mask mark ispositioned in the range from the c-th row to d-th row. It is assumedthat a q-th column traverses between the partial two-dimensional image42A of the wafer mark and the partial two-dimensional image 43 of themask mark, and that a p-th column traverses between the partialtwo-dimensional image 42B of the wafer mark and the partialtwo-dimensional image 43 of the mask mark.

Image signals of pixels in the range from the first column to the p-thcolumn and in the range from the (q+1)-th column to the last column aresynchronously accumulated from the a-th row to the b-th row. Synchronousaccumulation means that image signals of pixels in the same column fromone row to another row are accumulated. Next, image signals of pixels inthe range from the (p+1)-th column to the q-th column are synchronouslyaccumulated from the c-th row to the d-th row. The image signalsobtained by two synchronous accumulations are synthesized to obtain asingle one-dimensional synthesized image signal.

FIG. 13 shows an example of the one-dimensional synthesized imagesignal. The abscissa of FIG. 13 represents a pixel column number, andthe ordinate represents a light intensity in a relative scale. Threesharp peaks corresponding to the partial two-dimensional image 43 of themask mark appear at the central area of the graph shown in FIG. 13, andthree sharp peaks corresponding to each of the partial two-dimensionalimages 42A and 42B of the wafer mark appear on both sides of the threesharp peaks in the central area. Steps are formed at positionscorresponding to the p-th column and q-th column. This is because theimage signals of pixels in the range from the a-th row to the b-th roware synchronously accumulated in the range from the first column to thep-th column and in the range from the (q+1)-th column to the lastcolumn, whereas the image signals of pixels in the range from the c-throw to the d-th row are synchronously accumulated in the range from the(p+1)-th column to the q-th column.

An S/N ratio can be improved by performing synchronous accumulation ofimage signals of a plurality of rows. Next, to what degree the S/N ratiocan be improved will be described.

FIG. 14 is a cross sectional view of the wafer and mask in the regionnear the object plane, and is similar to FIG. 7C.

As shown in FIG. 14, an angle of the optical axis 125 of the opticalsystem 120 relative to the direction normal to the exposure surface isrepresented by α. Images of edges of the mask mark 14 can be focussedclearly upon the light reception plane 129B if the edges are positionedin the range of the depth d of focus. The width L of an image of thisfocussing range vertically projected upon the object plane 27 is givenby:

L=d/tan(α)  (10)

If the high magnification factor of an image focussed on the lightreception plane 129B shown in FIG. 12A is represented by N and the pixelpitch in the column direction on the light reception plane 129B isrepresented by P, then the number (d−c+1) of rows traversing a clearimage portion 43 is given by:

d−c+1=L/(P/N)  (11)

The equations (9) and (10) are substituted in the equation (11) so thatthe following equation is obtained:

d−c+1=N×λ/(NA²×P×tan(α)  (12)

In the position alignment of this embodiment, N=100, λ=0.6 μm, NA=0.35,P=13 μm and α=30°. These values are substituted into the equation (12),leading to the number (d−c+1) of rows of nearly 65. Synchronousaccumulation of image signals of 65 rows improves the S/N ratio by about65^(½) which is nearly an eightfold.

By using the waveforms shown in FIG. 13, the relative position of themask and wafer marks can be detected by the method similar to thatdescribed with FIGS. 9A and 9B. The distance between centers can beobtained by interpolation calculation at a precision of (P/N) μm orshorter. With this high precision position alignment, a precision of+/−2 to 3 nm is possible.

In the fourth embodiment, while the synchronous accumulation for thepixels in the range from the a-th row to the b-th row of the lightreception plane 129B shown in FIG. 12A is performed, the pixels in therange from the (p+1)-th column to the q-th column are not synchronouslyaccumulated. Further, while the synchronous accumulation for the pixelsin the range from the c-th row to the d-th row is performed, the pixelsin the range from the first column to the p-th column and in the rangefrom the (q+1)-th column to the last column are not synchronouslyaccumulated. It is therefore possible to shorten the time required forthe synchronous accumulation.

In the fourth embodiment, although the area subjected to the synchronousaccumulation has been determined as above, other partial areas may besubjected to the synchronous accumulation. In such a case, the partialareas subjected to the synchronous accumulation are decided so that thepartial two-dimensional images 42A and 42B of the wafer mark and thepartial two-dimensional image 43 of the mask mark overlap at least atportions of accumulation areas. By deciding the accumulation area in theabove manner, the calculation time can be shortened and positionalignment can be performed at a high precision because the S/N ratio canbe improved.

Also in the fourth embodiment, two images of the two wafer marks and oneimage of one mask mark are focussed upon the light reception plane.Instead, two images may be formed on the light reception plane to detectthe relative position of the two images. In this case, the relativeposition of the two images can be detected from the image signalsobtained through synchronous accumulation of image signals in the columndirection in the two areas each of which overlaps at least a portion ofeach of the two images.

In the above-described embodiments, position alignment of a wafer and amask has been described. The above embodiments are not limited only tothe position alignment of a wafer and a mask, but are applicable also tothe position alignment of a first member having a main surface and asecond member disposed apart from the main surfaces by a certain gap.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent that various modifications, improvements,combinations, and the like can be made by those skilled in the art.

What is claimed is:
 1. A position detecting method comprising the stepsof: disposing a member with an exposure surface to be exposed and anexposure mask in proximity with each other, with a gap being set betweenthe exposure surface and the exposure mask, the member having analignment mark formed on the exposure surface, the alignment mark havingedges or apexes for scattering incident light, the exposure mask havinga mask mark formed on a surface thereof, the mask mark having edges orapexes for scattering incident light; and detecting a relative positionof the member and the exposure mask by applying illumination light tothe edges or apexes of the alignment and mask marks, by focussing lightscattered from the alignment and mask marks on a light reception plane,and by observing images on the light reception plane, whereby a lightflux scattered from one of the alignment and mask marks is attenuated orlight fluxes scattered from both of the alignment and mask marks areattenuated differently, so that a light intensity of an image formed bythe light flux scattered from the alignment mark and focussed on thelight reception plane becomes near to a light intensity of an imageformed by the light flux scattered from the mask mark and focussed onthe light reception plane.
 2. A position detection method according toclaim 1, wherein in said relative position detecting step, lightscattered from the alignment mark is focussed via a first opticalfilter, and light scattered from the mask mark is focussed via a secondoptical filter having a transmission factor different from atransmission factor of the first optical filter.
 3. A position detectionmethod according to claim 2, wherein the first and second opticalfilters are defined in different areas of one optical filter member. 4.A position detection method according to claim 3, wherein the opticalfilter member is disposed just in front of the light reception plane. 5.A position detection method according to claim 1, wherein in saidrelative position detecting step, scattered light is focussed on thelight reception plane by using an optical system having a same opticalaxis as an optical axis of the illumination light.
 6. A positiondetection apparatus comprising: an illumination optical system forapplying illumination light to a member having an exposure surface to beexposed and to a mask disposed in parallel to the exposure surface ofthe member and spaced by a gap from the exposure surface; and anobservation optical system which has an optical axis oblique to theexposure surface of the member and which focusses light scattered fromthe member and the mask onto a light reception plane, wherein saidobservation optical system comprises an optical filter disposed just infront of the light reception plane, and a transmission factor of theoptical filter is different between an area corresponding to an areawhere light scattered from the member is focussed and an areacorresponding to an area where light scattered from the mask isfocussed.
 7. A position detecting apparatus according to claim 6,wherein the optical filter comprises a neutral density filter definingtwo areas having different transmission factors.